As digital cameras, digital video cameras, HDD (Hard Disk Drive) recorders and the like have become widely used, digital contents owned by persons have increased in amount. In these circumstances, there are known content retrieval technologies for providing users with contents which the users desire to use, from among a large amount of contents. One of the content retrieval technologies is a method for attaching, to the contents, metadata that represents attributes of each content, the attributes including, for example, a content creation date/time, location, creator's name, and character string representing the substantial content of a content, and performing searching by using the metadata (see Patent Literature 1).